Opioids are powerful painkillers that act on the brain, but they have a range of harmful side effects including addiction. Researchers from the Max Planck Institute of Biochemistry (MPIB) in collaboration with researchers from the Medical University of Innsbruck, Austria, University of Innsbruck, and the Lewis Katz School of Medicine at Temple University (LKSOM), have developed a tool that gives deeper insights into the brain’s response to opioids. Using mass spectrometry, they determined changes of proteins’ phosphorylation patterns—the molecular switches of proteins—in five different regions of the brain and assigned them to the desired and the undesired effects of opioid treatment. Their results, which are published in the journal Science, will lead the way for identification of novel drug targets and design of a new class of painkillers with fewer side effects.